Energy-efficient and high-intensity light sources such as LEDs (light emitting diodes) or lasers, usually in the form of laser diodes, are increasingly being used nowadays in modern lighting devices. Unlike incandescent bulbs, which are thermal emitters, such light sources emit light in a narrowly delimited spectral range such that their light is almost monochromatic or exactly monochromatic. One possibility for opening up further spectral ranges consists of light conversion, for example, wherein phosphors are irradiated by LEDs and/or laser diodes and for their part emit light having a different wavelength. By way of example, a layer comprising phosphor can be illuminated by LEDs or laser diodes and for its part emits light having a different color, i.e. a different wavelength. By way of example, this technique can be used to convert light of blue LEDs into white light by admixing yellow light generated by excitation of a phosphor-containing layer.
As conversion layers, thin phosphor layers such as silicate minerals, orthosilicates, garnets or nitrides are applied to surfaces of corresponding carriers. In that case, the phosphor layers are usually mechanically fixed with binders and linked to an optical system (lenses, collimators, etc.), wherein light coupling can be effected via air or by an immersion medium, for example. To ensure that the optical system is optically linked to the phosphor as optimally as possible and to avoid light losses, as direct an optical linking as possible should be ensured.
In the applications mentioned above, the phosphors are usually excited to emission by LEDs and/or laser diodes having high light powers. Thermal losses that arise have to be dissipated, for example, via the carrier to avoid overheating and thus thermally governed changes in the optical properties or even destruction of the phosphor.
Without an additional use of binders, for example, silicones, the phosphors usually present in pulverulent form do not form mechanically stable layers, i.e. abrasion- and/or scratch-resistant layers. However, binders are also generally used to combine the phosphor particles into one phase which can then be applied to corresponding surfaces. With the use of binders for layer stabilization, however, those binders can interact with the phosphors and thus adversely influence their optical and thermal properties, and also their lifetime. Furthermore, thermal conductivity of the binders often constitutes a limiting variable in dissipation of heat that arises in the converter element.
As alternatives, converter elements are known which are formed from a ceramic comprising the phosphor or from a crystal comprising the phosphor. In particular, the phosphor can form the ceramic or the crystal. Such converter elements can be fixedly adhesively bonded to LEDs such that the heat that arises therein can be dissipated. In that case, a limiting variable for the heat dissipation is the thermal conductivity of the adhesive used. Furthermore, it is beneficial to good heat dissipation if the converter elements are made particularly thin. However, a limiting variable for the thickness of the converter element is the stability of the converter element, the stability diminishing with diminishing thickness, and the required handlability when applying the converter element to the heat sink. In very thin converter elements, this can lead to a high level of rejects in the production process. The phosphor used is embedded in the ceramic or incorporated in the crystal structure and, in various examples, can be a phosphor mixture comprising a mixture of different phosphors as a result of which light which combines a plurality of different colors can be generated, for example. The converter element can consist, for example, completely or only partly of crystal or ceramic. Independently thereof, the converter element can comprise a matrix material which can comprise diamond or Al2O3, for example.
Furthermore, in electromagnetic radiation emitting assemblies, which hereinafter are also designated as assemblies for short, electromagnetic radiation emitting components which hereinafter are also designed as components for short, are embedded into a potting material comprising the phosphor or phosphors. In other words, the phosphors are embedded into the potting material in these assemblies. The heat that arises during the light conversion then has to be dissipated via the potting material.
Important parameters of the LEDs are the lifetime and brightnesses thereof. To attain high brightness values, the LEDs are regularly operated with relatively high energies which can lead to a high operating temperature since, during the conversion process, much unused energy is converted into heat. As the operating temperature increases, however, the lifetime decreases since the corresponding assembly becomes more susceptible to cracks and/or can age more rapidly. To ensure good and rapid heat dissipation, conversion layers are arranged in part near the corresponding components such that the heat that arises in the conversion layers can be rapidly dissipated via the corresponding component. The conversion layers near the components, for example, high-power LEDs, can be formed, for example, by EPD (electrophoretic deposition), spray coating and/or layer transfer, for example, the transfer of ceramic or silicone laminae or layers.